1. Field of the Invention
The present invention relates to a light modulator integrated light-emitting device and a method of manufacturing the same, and more specifically to improvement of a semiconductor device, in which a semiconductor laser element and a light modulator are integrated, for using in an optical communication system.
2. Description of the Related Art
As a distance gets longer and capacity gets larger in an optical communication system, a transmission characteristic of 2.5 Gb/s is required in a communication line of 100 Km or longer, it is expected in the system that a light modulator integrated light-emitting device is put to practical use.
A light modulator integrated light-emitting device such as described in the JOURNAL OF LIGHTWAVE TECHNOLOGY [M. Suzuki et al., Vol. 6, No. 6, June 1988] has a light-emitting element 2 and a light modulation element 3 provided adjacent to the light-emitting element 2 on a semiconductor substrate 1 having a common electrode 1A as shown in FIG. 1. The light-emitting element 2 has a laser electrode 2A for supplying an electric current for generating the laser light, and the light modulation element 3 has a modulation electrode 3A for supplying a signal for laser modulation.
Next, the operation of the light modulator integrated light-emitting device will be described. First, when a driving current is supplied to the light-emitting element 2 from a constant current source connected between the common electrode 1A and the laser electrode 2A, a laser light is generated in the light-emitting element 2. Further, when a modulation signal is supplied to the modulation electrode 3A from a signal source connected between the common electrode 1A and the modulation electrode 3A, the laser light from the light-emitting element 2 is intensity-modulated by the Franz-Keldysh effect. The modulated light with the above is propagated to an optical fiber or the like from the device concerned.
Now, in a light modulator integrated light-emitting device according to a prior art, the boundary region between the light-emitting element 2 and the light modulation element 3 is separated electrically by means of a semi-insulating semiconductor, and the resistance of this separated region is normally several K.OMEGA. or higher.
However, when a separation distance between the laser electrode 2A of the light-emitting element 2 and the modulation electrode 3A of the light modulation element 3 gets shorter as micronization and integration of the semiconductor device advance, electrical separation gets weaker. As a result, electromagnetic induction is generated between the laser electrode 2A and the modulation electrode 3A, and a high frequency signal induced into the laser electrode 2A from the modulation electrode 3A exerts a bad influence upon the constant current supplied to the laser electrode 2A. For example, the current supplied to the light-emitting element 2 varies, fluctuation is caused in the oscillation wavelength of the laser light, and the modulated light becomes unstable.
The foregoing is an obstacle for long distance and large capacity transmission in an optical communication system, thus lowering the reliability. Besides, the more the modulation signal having a high frequency is supplied to the modulation electrode 3A, the more easily the electromagnetic induction is generated. The electromagnetic induction is also affected by a bonding wire connecting respective electrodes 2A and 3A with input terminals, arrangement of respective electrodes and the frequency of the modulation signal.